An image data subtraction system suitable for use in Angiography or other medical procedure enhances vessel visualization. An imaging system subtracts data representing a first mask image from data representing temporally sequential individual images of patient vessels to provide first digitally subtracted images. The sequential individual images encompass introduction of a contrast agent. An image processor automatically, processes the data representing the first digitally subtracted images to identify a first image indicating presence of the contrast agent and a second image preceding the first image by determining a measure representing a net presence of contrast agent in an individual digitally subtracted image. The second image is substantially exclusive of an indication of presence of the contrast agent. The image processor automatically, selects the second image as a second mask image and subtracts data representing the second mask image from the data representing images of the temporally sequential individual images to provide second digitally subtracted images. A user interface presents the second digitally subtracted images for display while the patient is undergoing a medical procedure.